Abstract

Multilocus DNA fingerprinting was used to estimate the frequency of extra-pair fertilizations in a low density, island population of Great Tits Parus major. A total of 69 pairs and 516 offspring from 82 breeding attempts were examined. Only 18 offspring (3.5%) in seven different nests were not fathered by the attending male. The sample included one brood in which all nine chicks were fathered by an extra-pair male. One chick from a nest of eight was the result of intra-specific brood parasitism. Three chicks from a brood of nine could be matched with the male but not with the female. Observations at this nestbox suggested that mate switching had occurred during the laying period. The percentage of extra-pair fertilizations did not differ between first clutches and (experimentally induced) replacement clutches. Females mated to small males were more likely to have extra-pair young in the nest. Because both extra-pair paternity and intraspecific brood parasitism are rare in this population, a reliable measure of reproductive success can be obtained by counting the number of offspring. [KEYWORDS: Intraspecific brood parasitism; mixed reproductive strategies; conspecific nest parasitism; eastern bluebirds; genetic similarity; female preference; sperm competition; hatching failure; sexual selection; tree swallows]